Path: utzoo!attcan!uunet!mcsun!ukc!mucs!m1!bevan
From: bevan@cs.man.ac.uk (Stephen J Bevan)
Newsgroups: comp.lang.lisp
Subject: Help with Macros [longish]
Message-ID: <BEVAN.90Sep21154633@panda.cs.man.ac.uk>
Date: 21 Sep 90 14:46:33 GMT
Sender: news@cs.man.ac.uk
Organization: Department of Computer Science, University of Manchester
Lines: 40

I've recently been reading a book on Scheme (The Scheme Programming
Language - R. Kent Dybvig) and in it, it uses a function
`record-case'.  This is similar to `case' except that it does
destructuring.  So for example I could define the following function
which given some expressions as lists, evaluates them.

(defun eval-expr (x)
  (record-case x
    (add (x y) (+ x y))
    (sub (x y) (- x y))
    (mul (x y) (* x y))
    (div (x y) (/ x y))
    )
  )

>(foo '(add 3 4))
7
>(foo '(mul 4 5))
20

This notation is particularly nice if you are writing simple
evaluators for languages.

Here's the problem.  I've tried to implement this in Lisp myself, but
to no avail.  I've RTFM on macros* but I can't seem to get the
arguments to evaluate at the correct time.  So I'm hoping some kind
soul will show me how this is done.  If you are a real masochist I can
even send you the code I've written so far (all 53 lines of it!), so
you can point out what is wrong with it.

Yours a struggling Lisper,

Stephen	J. Bevan		bevan@cs.man.ac.uk


* if fact I've got all of the following in front of me
    Common Lisp, Steele et al.
    Common Lisp - A Tutorial, Wendy L. Milner
    Lisp, Winston & Horn
  but I'm still can't get the parms. to evaluate when I want them to.